The present invention relates generally to machining, and, more specifically, to Computerized Numerical Control (CNC) machining of a cellular workpiece.
Honeycomb structural panels are commonly used in the aerospace industry in fabricating aircraft due to the high strength thereof and low weight. Such panels can be formed of various materials including synthetics and metals, and typically have a honeycomb core in sheet form bounded by opposite skins adhesively bonded thereto.
In one exemplary application, an annular aircraft engine inlet is formed with small honeycomb core cells for effecting sound attenuation, with a structural outer skin, and an optional inner skin which would be perforated.
The honeycomb core is initially obtained from various manufacturers in various configurations and materials, and is typically provided in large rectangular stock sheet form, such as 2 meters by 3 meters. The core itself may be relatively flexible without its inner and outer skins, and typically includes a multitude of individual core cells due to its initially large stock configuration.
The core is typically identified by its nominal core size, such as ⅜ inch (9.525 mm), and for the 2×3 meter stock size can have about 210 by 315 cells for a correspondingly large number of total cells in a single core sheet, with the distribution of cells being generally uniform in the multiple columns and rows.
Honeycomb core sheets having exemplary hexagonal cells formed from synthetic, high strength Aramid fibers typically include continuous Aramid ribbons extending in serpentine form along the ribbon or longitudinal direction or axis, with laterally adjoining ribbons being spaced apart in the transverse direction or axis perpendicular to the ribbons and their longitudinal axis. For exemplary hexagonal cells, adjacent ribbons share common walls suitably bonded or fused together.
The length of the ribbons determines the corresponding longitudinal length of the resulting core sheet, and the number of adjoining ribbons determines the lateral width of the core sheet, which is typically larger than the length thereof.
In the exemplary aircraft engine inlet configuration, the stock honeycomb core sheets are initially purchased from a commercial vendor, and then undergo suitable fabrication processes to achieve the desired inlet configuration. One exemplary configuration requires the machining of a small notch or slot in the shared wall of adjacent hexagonal cells for the full number of small cells in the inlet configuration, which number is quite large.
Conventional processes may be used to machine such cell slots, but can experience various problems which undesirably increase the complexity, duration, and cost of machining.
One conventional process considered during development includes the use of a conventional multi-axis CNC router. The typical router includes a work table upon which the workpiece may be placed.
The router includes a gantry supporting a carriage atop the table, with suitable linear drive systems to effect compound orthogonal X-Y movement of the carriage which supports a spindle to which various router bits may be attached.
The spindle includes a third or vertical Z-axis for lowering or raising the router bit atop the mounted workpiece which is suitably machined according to its specific geometry and machining requirements.
The typical CNC router includes a programmable computer and a cooperating CNC controller. The computer provides a graphical user interface (GUI) for the operator and may be programmed in software for machining predetermined workpieces in predetermined configurations, typically in repetitive predetermined machining processes for a large quantity of identical workpieces.
The predetermined machining configuration is created during development and testing for specific workpieces, and a correspondingly specific machining program is defined for use in the CNC controller. The CNC controller is specifically configured for the particular type of CNC machine in controlling motion of the machine tool cutter along the several axes, including X,Y,Z, for example.
Conventional Numerical Control (NC) codes, such as known G-codes or ISO-codes, are used in the CNC controller to control the X,Y,Z motions of the corresponding linear drive systems, and thereby move the cutting tool linearly along each axis for collectively effecting compound X,Y,Z movement of the cutting tool as desired.
The CNC program code is therefore specific to the specific workpiece and its size and configuration, and the desired machining thereof.
Since the exemplary large honeycomb core sheet addressed above includes a multitude of small core cells specially arranged in columns and rows, it would therefore require suitable fixturing atop the router table and a suitable predetermined program code specific to its large length and width, and cell size, configuration, and relative positions.
For example, the predetermined program code could be developed to cut the desired slot in each cell with a router bit travelling along the middle of each row of 315 cells in 210 passes corresponding with the 210 columns; provided, of course, that the core sheet was precisely uniform in both the lateral and longitudinal directions, and accurately mounted atop the table.
For different sized workpieces with different sized cells with different configurations, a different program code would be required. Indeed, any change in configuration of the specific workpiece would necessarily require a different program code developed specifically therefor. And, larger size workpieces increase the likelihood of drifting of the cut in a correspondingly long machining path due to small variations in workpiece placement atop the table.
Fundamentally, the desired stock honeycomb Aramid core sheet is quite large and includes a large multitude of individual cells which would therefore require suitable mounting and alignment in a conventional cutting machine. The alignment method must be specific to the specific core type and size, and therefore can vary as workpieces vary.
Since the machine is controlled by an operator, the entire machining process can be labor intensive to ensure that mechanical alignment of the core sheet is maintained.
The core cells may shift out of position into misalignment under the forces of machining, and cause the machine to divert or drift from the intended cut path or crush the core cell walls causing irreparable damage to the core sheet, especially problematic for the large size of the stock core sheet and multitude of cells.
Conventional clamping around the perimeter of the stock sheet may lead to substantial scrap of the periphery, and further increase costs.
And, the need to suitably clamp or stabilize the stock sheet and machine the multitude of individual cells therein can result in correspondingly long machining or processing time, which further increase the cost of manufacture.
Accordingly, it is desired to provide an improved method of machining a cellular core workpiece having a multitude of individual cells subject to variations in configuration.